In refinery operations such as a vacuum distillation unit and similar services, the degree of vacuum separation attainable may be limited by various factors and concerns. One concern is for excessive coking of the vacuum column charge heater. To avoid excessive coking, limits may be placed on the system, such as a limit on the outlet temperature of the charge heater, or a limit of a specified degree of vaporization at the heater outlet. Limiting the outlet temperature of the charge heater relates to the time-at-temperature that the liquid film on the process side of the heater tubes experiences, with higher time-at-temperature being correlated to increased coking/fouling. The liquid film temperature can be significantly hotter than the bulk process temperature. Limiting the degree of vaporization at the heater outlet relates to the propensity for a dry-point to form inside heater tubes, wherein the liquid film vaporizes and comes to an end. As the liquid film shrinks and ceases to exist, both its temperature and the heat flux through it increase until it exits the charge heater, vaporizes, or cokes on the heater tubes. Such limits on the vacuum column charge heater outlet temperature or degree of vaporization can negatively impact the amount of lift attainable in the vacuum unit.
In other services, the process may be even more sensitive to such limits. For example if a process involves separation of a mixture of 80 wt % light hydrocarbons and 20 wt % heavy hydrocarbons, the processing may be severely restricted by a vaporization limit. If the heavy hydrocarbons are very heavy, such as streams containing a significant amount of heavy poly-aromatics, the charge heater outlet temperature might also be limited to avoid exposing these components to excessive time-at-temperature.
Furthermore, in existing processes, the feed to a vacuum distillation unit has typically already been through distillation at atmospheric pressure in a refinery's crude distillation unit (CDU). As a result, simple inclusion of a flash drum upstream or downstream of the vacuum column charge heater would not be justified. Flash drums are usually included in a design to remove non-condensables or components substantially above their critical points as to be clearly located in the vapor phase. The amount of such components present following atmospheric distillation is insufficient to justify inclusion of the flash drum and associated equipment downstream of the vacuum column charge heater. In addition, the inclusion of a flash drum downstream of the vacuum column charge heater would have other negative effects including the loss of vapor traffic to the column and material that would act in a stripping service, decreased resolution of product separation, and the requirement of a higher heater outlet temperature, which is undesirable with regard to feed cracking and coking, and the same limits described previously. The disadvantages of such configurations apply whether or not the flash drum is coupled to the vapor space in the vacuum column or to the vacuum column overhead.
Therefore, there is a need for improved vacuum separation processes with high vaporization.